1. Field of the Invention
The present invention relates to a scroll type compressor for use in a vehicle's air conditioning system. More particularly, this invention relates to a mechanism for maintaining the dynamic balance of a movable scroll and its associated members while a compressor is running.
2. Description of the Related Art
Generally speaking, the operation of a scroll type compressor uses the revolving movement of a movable scroll angularly interfit with a fixed scroll inside the housing of the compressor to compress refrigerant gas. Each of the fixed and movable scrolls has a spiral element and a fixed end plate. When interfit with each other, the two scrolls form gas pockets. When the movable scroll revolves relative to the fixed scroll, the pockets spiral with decreasing volume toward the center of the scrolls, thereby compressing the refrigerant gas.
Operational power is transmitted to such compressors via a rotary shaft supported by a bearing in the front of the compressor housing. An eccentric pin, attached to the end of the rotary shaft, projects into the front end of the compressor housing. A boss, formed on the front face of the movable scroll's end plate, fits over the eccentric pin via a bushing and a bearing. This allows the movable scroll to rotate relative to the eccentric pin.
An anti-rotation device, between the movable scroll and pressure receiving wall of the housing on the fixed scroll side, inhibits the movable scroll's rotation. The anti-rotation device does however allow the movable scroll to revolve around the axis of the rotary shaft. A balance weight, attached to the eccentric pin, dynamically balances the rotary shaft and movable scroll against the centrifugal forces produced by the revolving movable scroll.
In conventional compressors, both the balance weight and the revolving movable scroll generate centrifugal forces which tend to oppose each other. In addition to these two forces, a compressive reactive force is generated on the movable scroll, during the compressor's gas compression stroke. This reactive force, in general, is not canceled by the centrifugal force set up by the balance weight. Consequently, the reactive force tends to be absorbed by the eccentric pin, the bearing and other structures supporting the movable scroll and contributes to their deterioration.
The actual weight of the balance weight also affects the compressor's performance. Acceptable design tolerances of the balance weight requires its weight to fall within three percent of the combined weight of the movable scroll and bushing weight. This is important since the weight of these components directly effects the centrifugal force produced by the movable scroll. Should the weight of the balance weight cause an increase in the centrifugal force, even by as little as 2%, the outer wall of the movable scroll's spiral element tends to separate from the inner wall of the fixed scroll during the movable scroll's revolution. This impairs the efficiency with which the gas pockets are sealed, reduces the compressor's efficiency and raises the temperature of the refrigerant gas.
A further disadvantage of conventional balance weights is their size. Large heavy balance weights inevitably require compressor housings with increased volumetric capacities. This, unfortunately, precludes the design of compact sized compressors.